2 Answers
2

The power going into an empty microwave will accumulate as an EM field inside the cavity up to the point where the input power equals the rate of leakage. Energy inside the cavity will leak out through the little holes in the metal screen in the door, through ohmic losses in the sided of the microwave, feedback into the power source, etc. The rate of power leakage from the cavity is given by $P_{leak} = 2\pi E f / Q$ where $E$ is the energy stored in the cavity, $f$ is the resonant frequency of the cavity and Q is the "quality factor". In equilibrium then, $E = QP_{input}/{2\pi f}$. For an resonant frequency of around 2.5 GHz, input power of 1kW, Q of say 100 (guess), the accumulated energy is only about a milli-joule. Of course, the materials making up the microwave probably aren't designed to dissipate 1kW of power over an extended time, so eventually the microwave may explode or something due to overheating of components.

I seem to recall hearing that the power would mainly return to the magnetron and cause internal arcing.
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Kevin ReidAug 16 '11 at 0:22

Thanks for a good explanation, but... Can you explain a bit where this equation came from? Like why would it radiate more with a higher resonant frequency? Did you find it somewhere? Aren't the bars on the front spaced to block a specific wavelength of light?
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user420667Aug 18 '11 at 2:50

See the wikipedia page on the quality factor: en.wikipedia.org/wiki/Q_factor . The mesh in front is designed to block the microwaves while letting visible light through, but it won't be 100% efficient.
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user1631Aug 18 '11 at 18:03

I'm not sure this answers the question. Probably many people can intuitively understand conservation of energy, but the question is, where exactly does this go if not the food? This answer proposes some guesses, but doesn't tell us if any of these are really significant.
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Peter MMay 14 '14 at 21:15

The microwave would blow up, eventually. If you read the user manual, some warning will placed about not operating the microwave when it's empty. Being a vegetarian, I have no idea about the bacon, but I can guess that it won't behave in a different way than any other material placed in the microwave. In small amount (single or double dish (?)) the energy absorbency is roughly linear and the heating time should be doubled.